X-y position detectors generally as described above are shown in U.S. Pat. No. 3,449,516. There the resistive layer is created by spraying on several layers of graphite film. Such a resistive layer is easily scratched, and the scratches influence the resistance value of the layer, so that it is no longer guaranteed that the component voltage tapped by the pen is analogous to its actual x-y position on the resistive layer. The device thus soon becomes unfit for use. In order to minimize the scratching which is almost unavoidably caused by the pen due to its movement, the ideal of preparing the tip electrode from a soft material, for example, copper, might be suggested. However, the pen tip would be worn off over time due to friction between it and the resistive layer. Moreover, the material worn off also influences the resistance value of the layer, leading to further inaccuracies.
The method of forming the resistive layer according to U.S. Pat. No. 3,449,516 does not provide an adequate smooth surface, because of the multi-layer spraying technique used. This promotes scratching and makes accurate measurement difficult, because along the path of its movement the pen will "jump" over the roughnesses, separating it from the resistive layer. This causes additional contact noise.
A similar device is described in British Pat. No. 12 80 341. In this device the resistive layer is said to consist of a hard wear-resistant material, such as a conducting plastic or a conducting ceramic, or an electrically resistive paper. Such materials can be wear-resistant due to a surface toughness. However, their hardness and smoothness are low, so that surface-generated noise as discussed above can occur in this case as well. In addition, the friction between such a material surface and the pen is high, so that a hard pen slides only with difficulty.
In similar devices, such as shown in West German Auslegeschrift No. 15 49 881 and U.S. Pat. No. 3,497,617, the resistive layer is covered by a hard insulating layer for its protection. The voltage component is then coupled to the electrodes capacitively, rather than galvanically, as in the case of the device of the type discussed above. It is undesirable to apply this additional protective layer over the resistive layer, because fluctuations in the thickness of the protective layer lead to errors in the measurements. In addition, the preparation of the protective layer involves additional cost. Furthermore, capacitive coupling of component voltages of the resistive layer leads to leakage of electric fields in the vicinity of the contact plate, which can lead to errors in voltage measurements.
A process by which a resistive layer with a very smooth surface can be prepared is described in West German Pat. No. 738,414. In that process, the resistive material used to form the resistive layer is applied to an intermediate substrate having a polished surface. After hardening, the resistive layer is attached to its surface on the side opposite the intermediate to a second substrate and the smooth intermediate substrate is removed from the other side. This process is called the reverse laminating technique. Further improvements on the reverse laminating technique are described, for example, in West German Offenlegungsschrift Nos. 30 31 751 and 31 35 554. Resistive layers prepared according to the reverse laminating technique are suitable for use as contact surfaces on which a moving contact slides in potentiometers. The moving contact slides on the potentiometer contact surface with a predetermined contact pressure which need not be excessively high. Therefore, the fact that the hardness of the potentiometer contact surface as prepared using the reverse laminating technique is rather low causes no problems. Such a potentiometer contact surface is not suitable as a resistive layer on which a manually moved pen is moved directly, because due to its relative softness it would become scratched within a short period of time.
Processes for preparing film resistors are described in West German Pat. No. 26 15 785 and in West German Pat. No. 27 20 615. According to those methods, very finely-ground carrier particles are coated with a metal. The coated carrier particles are then coated in carbon, by a pyrolysis process. These particles are then mixed with a binder. The resistive paste thus obtained is applied on a substrate. It was found that a resistive layer prepared using this process becomes extremely hard if the carrier particles consist of silica. Such hard resistive layers are only marginally suitable for potentiometer contact surfaces.